Rotating labyrinth seals have a wide variety of uses and one such use is to effect sealing between plenums at different pressures in gas turbine engines. Such seals generally include two principal elements, i.e., a rotating seal and a static seal or shroud. The rotating seal, in cross section parallel to the axial length of the engine, frequently has rows of thin tooth-like projections extending radially from a relatively thicker base toward the static seal or shroud. The static seal or shroud is normally formed from a thin honeycomb ribbon configuration. These principal elements are generally situated circumferentially about the axial length of the engine and are positioned with a small radial gap therebetween to permit assembly of the rotating and static components. The purpose of the labyrinth seal arrangement is to minimize gas path leakage out of the primary gas path and to segregate different stages of the compressor which are at different temperatures and pressures.
To a significant extent, engine efficiency depends upon minimizing this gas leakage around rotating components by controlling the gas flow to maximize interaction between the gas stream and the components in the primary gas path. The effectiveness of the turbine engine varies directly with the proportion of gas that impinges upon the blades of the rotating member. Closer tolerances between the rotating and static seals achieve greater efficiencies. The fabrication process to obtain these close tolerances is extremely costly and time-consuming.
When the gas turbine engine is operated, the elevated temperatures of operation cause the opposed static and rotating seals, such as those in the rotating labyrinth seals, to expand in a radial direction toward each other. The rotating labyrinth seals expand radially and rub into the shroud, creating frictional contact between the thin projections of the rotating seal and the shroud. During the rub, there is high thermal compression, with resultant high residual tensile stress after the rub. This frictional contact causes elevation of seal teeth temperatures in excess of 2,000° F. with resulting possible damage to one or both seal members. For example, rotating tips may crack and break off, significantly impairing the seal efficiency and operation of the engine.
The thin, honeycomb ribbon construction of the shroud is used to reduce the surface area on which the seal teeth rub while reducing the weight of the structure, and helps to minimize the heat transferred into the rotating seal, while also providing the required strength. In addition, the rotating labyrinth seal teeth tips are constructed so as to be thin, in order to thermally isolate them from the supporting base or shell structure. However, excessive heat from deep rubs (even into the honeycomb) during engine start-up and during engine excursions can damage the rotating knife edge seals, negatively affecting durability and engine efficiency and providing a leak path for the flow of gases. Furthermore, material transfer can occur which also degrades the seal characteristics. Cutting into even low-density honeycomb cells can still cause rotary seal tooth damage, leading to premature part retirement.
While much effort has been directed at improving the rotating structure of the seal arrangement, there is a continuous need for improved designs for rotating labyrinth seal structures including improvements directed to the static structure to increase both service life and engine operating efficiencies.